1. Field of the Invention
The invention relates to upgrading of light olefinic streams, ranging from ethylene containing streams up to naphtha boiling range materials, to produce high quality distillate and motor fuel.
Upgrading of light olefinic streams by oligomerization over acid-type catalysts has long been known. Solid phosphoric acid catalysts, or liquid phosphoric acid on a support, were used to polymerize propylene and butenes to gasoline boiling range materials. The acid required a certain amount of water, so contacting the feed with water was a convenient way to add water, and also to remove nitrogen compounds which were catalyst poisons. The nitrogen content of the feed had to be limited to less than 50 ppm basic nitrogen for satisfactory life of the phosphoric acid catalyst.
An extensive review of more modern oligomerization technology, and much experimental work, was reported by W. E. Garwood, in Conversion of C2-C10 to Higher Olefins over Synthetic Zeolite ZSM-5, ACS Symposium Series, No. 218, Intrazeolite Chemistry, Galen D. Stucky and Francis G. Dwyer, Editors, Copyright 1983 by the American Chemical Society.
Garwood reported that ethylene was far more difficult to convert than higher molecular weight olefins. At 400 psig, in a temperature range of 400-600.degree. F., at some temperatures 95% conversion of propylene in feed was achieved while only 5% conversion of ethylene in feed occurred.
Garwood used commercial grade olefin feed stocks, of greater than 95% purity. The effect of nitrogen contaminants was not discussed. The effect of hydrogen on yields and catalyst life was not investigated. Some experiments were conducted in the presence of hydrogen, but the hydrogen was used, with nitrogen, as a diluent so that the catalyst would see an olefin partial pressure of 1-2 psia while the total pressure was 1 atmosphere.
The patent literature also reports much work on converting lower olefins to gasoline and distillate.
U.S. Pat. No. 4,544,792, which is incorporated herein by reference, contains an extensive review of olefin oligomerization developments. This reference reported that moderate temperature and relatively high pressure could be used to favor aliphatic, distillate range product. Lower olefinic feedstocks containing C2-C8 alkenes could be converted, however, these conditions which favored production of relatively high boiling distillate do not convert a major fraction of ethylene.
In the upgrading of Fischer-Tropsch olefins, it has been found beneficial to water wash a C3-C7 olefinic stream to reduce the oxygenate content to below 1 Wt %, followed by oligomerization in a pressurized reactor in the presence of hydrogen. U.S. Pat. No. 4,544,792 reported that feedstocks containing trace amounts of oxygenates could be catalytically upgraded over H-ZSM-5 when as little as 1 mole % H2 was present. When increased oxygenate concentrations were encountered, more hydrogen was required. The cofed hydrogen inhibited the formation of coke on the catalyst.
A related approach to the problem of dealing with oxygenates in Fischer-Tropsch liquids is disclosed in U.S. Pat. No. 4,513,156, which is incorporated herein by reference. A more aggressive extraction of oxygenates was practiced, to remove essentially all of the oxygenates upstream of the olefin oligomerization reactor. A multi-stage vertical extraction columns was used for water washing or extracting oxygenates from the feed.
Upgrading of pyrolysis naphtha produced during steam cracking to make ethylene, by passing the naphtha over Pd/Zn/ZSM-5 at 900.degree. to 1200.degree. F. was disclosed in U.S. Pat. No. 4,097,367, which is incorporated by reference. The high temperature processing of the pyrolysis naphtha converted to aromatics everything boiling in the BTX range, yielding a liquid product with essentially no non-aromatic hydrocarbons boiling above 167.degree. F. The patentee discussed the general prejudice in these arts re. the presence of hydrogen, namely that ZSM-5 is known for conversion of olefins to aromatics, but preferably in the absence of hydrogen.
Refiners have been reluctant to subject olefinic streams to hydrotreating, or to catalytic hydroprocessing, out of concern that the olefins would be saturated. In gasoline boiling range materials this saturation would greatly decrease the gasoline octane. In C2 to C4 olefins, saturation of the olefins would reduce greatly their reactivity, and reduce the value of these materials. C2 olefins are very difficult to oligomerize, and C2 paraffins are even less reactive. The presence of large amounts of basic nitrogen in these streams makes hydtrotreating desirable, but the need to preserve olefins makes it unwise.
We reviewed the work that others had done, for guidance on how best to upgrade refinery olefin containing streams to heavier, more valuable products. Two types of problems are present, feed contamination and presence of materials which are not contaminants but a difficult to process.
Feed contamination is a severe problem with olefinic streams in a refinery because these streams usually contain large amounts of sulfur and nitrogen. Much of the nitrogen contamination is introduced in trying to reduce the sulfur contamination, i.e. amine scrubbing removes sulfur but leaves trace amounts of amine in the scrubbed stream.
The presence of basic nitrogen in the light olefins refinery streams can damage the zeolite oligomerization catalysts, so it would be beneficial if a way could be found to make these catalysts more tolerant of catalyst poisons which are always found in refinery olefin streams. A better feed pretreatment process is needed, which can reduce the level of nitrogen contaminants in the feed at least two orders of magnitude below the 50 ppm tolerated by the phosphoric acid olefin oligomerization process. The feed pretreatment process should also be able to tolerate momentary overloads and upsets, while still protecting the zeolite catalyst.
The feed pretreatment process should be one which can operate continuously, or at least for long periods using a regenerable, rather than a replaceable, nitrogen removal medium. Feed pretreatment can be further complicated by the presence of large amounts of unusual contaminants in refinery light olefin streams, especially the presence of large amounts of dienes. Some of these contaminants, probably the dienes, can irreversibly deactivate some nitrogen removing materials. Thus a conventional nitrogen sorbent which is regenerable in some other service might not prove suitable for use in feed pretreatment of light olefinic streams prepared by high temperature thermal and/or catalytic processing of crude oil.
Another problem with light olefinic streams from a refinery, besides the difficulty in pretreating such streams, is that refinery propylene frequently contains large amounts of ethylene. It would be beneficial if an upgrading process were available which could efficiently convert ethylene as well as heavier olefins.
It is also important to have a catalytic olefin upgrading process which can operate for a long time, at relatively high severity and at relatively high space velocities.
We have discovered a feed pretreatment process which provides an ideal way to efficiently remove basic nitrogen contaminants from refinery light olefin streams and make these streams suitable for downstream zeolite upgrading.
We have also discovered an efficient light olefin upgrading process, which can tolerate, e.g., the presence of large amounts of ethylene in a propylene rich stream, and which will efficiently convert a mixture of light olefins to heavier, more valuable normally liquid products. The olefin upgrading process tolerates the presence of relatively large amounts of dienes.